dt-bindings: pinctrl: Add bindings for Intel Thunderbay pinctrl driver

Add Device Tree bindings documentation and an entry in MAINTAINERS file
for Intel Thunder Bay SoC's pin controller.

Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
Link: https://lore.kernel.org/r/20211201072626.19599-2-lakshmi.sowjanya.d@intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>